In recent years, many motor vehicles have been designed in a manner that the drive shaft of the internal combustion engine can not be used to drive the cooling fan necessary for directing a convection air current across the radiator, which current is needed, especially when driving at slow speeds in hot weather. With the advent of the front wheel drive vehicles with transverse engines and limited air intake openings, many motor vehicles are now being manufactured with electric motors driving fans for cooling the radiators. These fans may be operated only when the radiator coolant temperature indicates a need for additional external cooling. Consequently, the cooling fans may be turned on only when the vehicle is stopped or operated in heavy traffic. These cooling fans may not be used for long periods such as during the depths of the winter in certain localities. Even with such diverse times when the fans are used, they must operate on demand. Otherwise, damage to the engine can result or the engine must be stopped immediately. Even though these fans are extremely critical, most drivers are unaware of their existence or their operation.
The motors driving these fans must operate after long periods of inactivity during which the motors may be subjected to adverse conditions, such as rain, salt from the road or physical obstructions. Thes fan motors are important and are relatively expensive. They can be damaged by high current flow through the armature winding when stalled or when shorted. A system is needed for automatically protecting the motors, even if stalled or shorted, from being subjected to continued activation which will result in permanent and costly damage to the motor, and in some instances, to the fan, operating relay and other components associated with the fan.
To operate cooling fan motors as described above, it is standard practice to provide a power relay which generally includes a printed circuit board with circuits for controlling the relay in response to a demand signal from the vehicle, i.e. when the coolant system requires external cooling by one or more fans. Such a power relay and printed circuit board containing the motor control circuits are relatively small and must be inexpensive since they are required by the millions by the automobile industry. In the past, these power relays included a coil controlled by a transistor circuit which placed the coil across the battery power supply in the vehicle whereby current flow through the coil would close normally opened contacts of the relay. When the fan was not needed, the transistor would interrupt current flow through the coil and the normally opened contacts of the relay would open. The combined power relay and associated printed circuit board are assembled into a single plug-in unit easily plugged into a socket in the engine compartment of a vehicle. Miniaturization and low cost are overriding requirements for this type of power relay and, as such, have limited the versatility of the known plug-in power relay units for operating the DC motors of a motor vehicle.
When using the standard plug-in power relays to control a motor driving a cooling fan, the relay contacts are closed and the power supply from the vehicle battery is applied across the motor. It has been found that a defective motor or a stalled motor can cause high current flow which will damage the motor. This causes one immediate disadvantage. A new fan motor is quite expensive. High current flow in the motor can also cause hazardous situations, such as combustion of the motor harness or other similar flammable situations. As can be seen, activating a stalled motor presents severe problems. If the motor is shorted, closing of the power relay contacts also causes extremely high currents to flow in the motor circuit with the same disastrous results. The high motor current, no matter what its origin, has a tendency to destroy the printed circuit board associated with the power relay and the associated control circuitry. Because of the miniaturization of the power relay for operating a motor in a motor vehicle, the criticality of the availability of the fan motor when needed, expense of replacing the fan motor when it is damaged and related demands on the plug-in power relay unit, there has been a need for some system to protect the motor from long term, high current operation without an increased size or cost or a decrease in reliability. The solution to these problems has generally involved the use of a circuit breaker, fuse or other device in the motor circuit to disconnect the circuit upon sustained, high current conditions. Such devices are slow in operation, need manual resetting and often are triggered when there is only a transistory condition in the motor itself. For instance, if the motor is stalled due to a physical obstruction, a protective fuse would trip due to the high current caused by the stalled condition of the motor. When the fuse trips, the motor is deactivated and will not operate even though the obstruction has been removed. Consequently, if the fuse operates and disconnects the motor, the engine may be operated without the fan during times when a fan is required. Also, circuit breakers and fuses are very slow and often drift in their detect range so that they have to have a relatively wide current band. Bimetal circuit breakers, series sensing elements and other standard arrangements for protecting the motor are equally susceptible to disadvantages mentioned above and, in many cases, are too big, expensive or complex for high production use on motor vehicles. All of these protective systems require a high current in the motor for detection; therefore, damage can result before the system has been actuated.
In some instances, a protective device such as a circuit breaker interrupts the current flow by opening the relay when high current is sensed. Then, the relay is closed and high current is again sensed to again open the relay. Repeated operation causes the relay contacts to erode or weld close. When a MOSFET or a bipolar device is employed as the power switch to the load, operation at high currents caused by a shorted or stalled motor can blow the FET or power transistor.
These same problems exist when a relay is operated to direct a high current to other DC loads such as lighting circuits of a motor vehicle.
As will be apparent, the invention is used to monitor load circuits used extensively in motor vehicles, such as a DC fan and blower motors, DC lighting circuits, DC motors for power windows and power seats, windshield motors, etc. Primary emphasis will be on a DC motor for the cooling fan since this presents a more critical situation threatening the actual operation of the vehicle; however, such emphasis should not distract from general use in other DC load circuits of the small types used in motor vehicles.